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Weathering and Soils
Fig. 5.21
v 0044 of 'Weathering and Soils' by Greg Pouch at 2011-01-24 11:41:38 LastSavedBeforeThis 2011-01-24 11:37:59
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Weathering and Soils
3 Weathering
4 Weathering > Mechanical Weathering
5 Weathering > Chemical
6 Weathering Products
7 Weathering > Rates
8 Differential Weathering
9 Soils
10 Why study soils
11 Vocabulary
12 Soil>Processes
13 Factors affecting soil formation
14 Soil Properties
15 Soil Classification
16 Soil Evolution
17 US Soils Fig. 5.18
18 Global Soil Regions USDA
19 Global Soil Regions FAO
20 Soils and History
Weathering mechanical or chemical destruction of rocks at or near the surface
•The surficial processes of weathering, erosion, transportation, and
deposition work together to generate sediments and sedimentary rocks,
as well as soils.
•Mechanical Weathering is breaking rocks into smaller rocks.
•Chemical Weathering is corrosion of rocks.
The near surface environment differs from deeper environments in the
abundance of water and oxygen and the presence of CO2, which
forms a weak acid when mixed with water. Most chemical
weathering is due to dissolution, oxidation, or hydrolysis.
•Erosion and Transportation are starting to move fragments (erosion)
and moving rock fragments (transportation) and are closely related to
mechanical weathering. When the transportation stops, we call that
Weathering > Mechanical Weathering
Mechanical Weathering is breaking rocks into smaller rocks.
• Water that gets into cracks then freezes can break up a rock
(potholes in Chicago), as can growth of plant roots and
formation of salt crystals through evaporation.
• If you cool a rock, all the originally interlocking grains get
smaller: this causes them to pull on each other, which results
in fractures (joints) developing in the rock, and even within
individual mineral grains. Similarly, if you un-load
(decompress) a rock, it expands in the free direction (towards
the surface) and can’t expand parallel to the surface, so you
end up with sheeting due to pressure release. In addition to
these planetary-scale causes, you also have daily temperature
fluctuations and forest fires that can cause rocks to fracture.
Burrowing animals can also fragment rocks
• Rocks striking each other knock or scrape fragments off: this
is abrasion. This is much more important after a fragment has
been freed and is being transported.
• Mechanical weathering of rocks increases the exposed surface
area, which favors chemical weathering, and decreases the
size of particles, which makes erosion and transportation
Fig. 5.8a
Weathering > Chemical
• Chemical Weathering is corrosion of rocks.
The near surface environment differs from deeper environments in the abundance of water and oxygen
and the presence of CO2, which forms a weak acid when mixed with water. Most chemical weathering
is due to oxidation, dissolution, or hydrolysis.
–Oxidation – Redox, Electron exchange Often is the addition of oxygen
• 4Fe+2
O2 →
4Fe+3+2O-2 (Iron is oxidized, Oxygen is reduced)
• 4FeSiO3
O2 → 4SiO2 + 2Fe2O3
• Fe-pyroxene + oxygen → silica + hematite (rust )
–Dissolution: Ions go into solution from a solid
• CaCO3 + H2O → Ca+2(aq) + HCO3-1(aq)
+ (OH)-1(aq)
• Calcite +water → Ca+2ion + bicarbonate ion +hydroxyl ion
N.B. Dissolving the calcite has “released” a hydroxyl, making the water more-basic/less-acidic.
–**Precipitation: Opposite of dissolution, ions taken from a solution to form a solid. This in not a
weathering but a depositional process.
• CaCO3 + H2O ← Ca+2(aq) + HCO3-1(aq) +(OH)-1(aq)
–Hydrolysis replacement of a cation (metal ion) with a hydrogen ion.
Hydrogen ions are very small. They can replace the normal cations in mineral structures at the
surface, which de-stabilizes the crystal lattice and enhances further reaction.
• 2KAlSi3O4 + 2 (H+ HCO3-)
+H2O → Al2Si2(OH)4 + 2K+ + 2HCO3- +4Si02
• K-Spar
+ (CO2 dissolved in water) +water → Kaolinite clay + soluble ions
+qtz (aq or s)
Weathering Products
• Smaller fragments of rock (due to mechanical weathering)
• Altered rocks due to chemical weathering. Different minerals.
aq_ means aqueous
s solution, or solid.
* indicates very slow or tropics-only
Mineral IN
Mineral OUT
Solute OUT
Calcite, Dolomite
HCO3 Ca Mg
Hydrolysis +
Precipitation goethite FeO(OH)
SiO2 (aq or s) Mg
extra H+
Pyroxene, Amphibole, Biotite
Hydrolysis + Clays
Precipitation FeO(OH)
SiO2 (aq_s) Mg,Ca,Na
extra H+
SiO2(aq_s) K, Ca, Na
AlO(OH) and FeO(OH)
SiO2(aq), other ions
Clays (very little change)
Solution *
• Bowen’s reaction series Minerals at the bottom of the series are the most stable under weathering, the ones
at the top are least stable.
• In order of increasing stability calc,dol<ol<pyr/amph/biot<plag<musc,kspar<qtz<clay<goethite<diaspore
Weathering > Rates
• Weathering rates are influenced by
– Heat,
– water,
– acids,
– ice,
– temperature changes,
– rock
• Most chemical reactions happen at the
surface of particles, so fine-grained
materials corrode faster.
• Being chemical reactions, all chemical
weathering proceeds faster at higher
• Reactions that involve water and/or
CO2 occur more rapidly in more humid
(vegetation-lush) conditions.
• In climates that have little or sporadic
rainfall and little vegetation (deserts),
mechanical weathering dominates and
you get angular topography (Western
movies) with minimal soil
development but soils that contain lots
of mineral nutrients.
• In climates with abundant rainfall and
vegetation and temperatures
(rainforests), chemical weathering
dominates and you get very rounded
topography and thick soils with little
mineral nutrients. (Most nutrients are
in biomass cycles).
Differential Weathering
It is rare for two different rock types to weather at the same rate. Rocks that are fractured
or made of reactive (unstable) minerals weather faster. Usually, differential erosion
occurs and is the main way that rocks are mapped.
Fig. 5.21
Why study soils
Plant nutrition
Heaving due to ice (rare this far south) or shrink-swell clay
Construction: settling of foundations
Useful information about recent geologic history
From Report: Urgent action needed to avert global
hunger (bad article but good map)
• Soil
– Engineer: any loose material that can be moved using a
backhoe, without blasting (includes what geologists call
unconsolidated sediment)
– Geologist: sediment that has plant roots
– Soil Scientist: loose material, including mineral and
organic matter, that supports plants
• Horizon a distinct, horizontal zone in a soil, often marked by
differences in mineral composition, texture, color, or structure
– O organic. Composed mainly of decomposed organic
matter (leaves and stems)
– A Accumulation zone. Mainly mineral matter, with
accumulated organic matter and mineral decomposition
products of plants, such as K, Ca, and Phosphate. Loses
clays and iron/aluminum oxides/hydroxides.
– E Eluviation zone (washed out zone). Clays and iron and
aluminum oxides are washed out of this zone, and nothing
else accumulates
– B Accumulation zone Clays and iron/aluminum oxides
accumulate here.
– C Slightly altered parent material, if from unconsolidated
– R Regolith (rotten rock) Altered rock, if from
consolidated rocks.
• Percolation of Water Soil is subjected to a continual flow of
water, which is slightly acidic due to CO2 in atmosphere, but
becomes more acidic due to high CO2 concentrations in soil along
with organic acids from decomposition of plant material.
• Weathering of minerals The same processes as occur in other
surface environments, but slightly more acidic due to
decomposition of plants
• Solution Soluble ions either leave completely or accumulate in B
horizon, depending on relative importance of rain and evaporation
• Precipitation Certain minerals are insoluble enough to
precipitate (phosphate, iron and aluminum oxides and
hydroxides). Others may precipitate because of dry conditions in
soil, or, in groundwater discharge zones, minerals precipitate from
the water that comes in from below and evaporates.
• Translocation (movement of colloids over short distances) Small,
solid chunks of clays and iron and aluminum oxides and
hydroxides can be moved in water, and deposited lower in the soil
column at permeability barriers
• Roots penetrate soil, increasing the permeability and porosity
• Other organisms burrowing animals aerate the soil, bacteria
decompose plant matter, can synthesize nitrate or release nitrate
from plants
• Decomposition of plants releases plants components back into
the soil
Factors affecting soil formation
• Parent Material
–Eolian (wind-borne) material. Sand or silt (loess)
–Fluvial sediments
• Climate
• Vegetation or Organisms
–Deep, thick roots, most of the biomass is above ground, soils with pronounced horizonation
–Roots relatively shallow, most of biomass is below ground, not as much horizonation, very
soft soils due to extensive roots (and holes where roots used to be)
• Topography
–Drainage status
• Time since exposure or deposition of material. Note that additional water or wind-borne
material may be deposited on top of the soil.
Soil Properties
•Texture (particle sizes)
•Structure: organization into peds (lumps of soil)
•Color (useful in soils)
–Black rich in organic matter
–Gray poor in organic matter and iron (hydr-) oxides Reducing =
poorly drained
–Brown, yellow, red iron Oxidizing
•Porosity percentage of volume occupied by air or water
•Permeability speed at which fluid flows through it in response to
pressure gradient, high (fast) in coarse materials, low in fine materials)
•Nutrient Availability Water, K, N, P, Ca, Mg, others
•Engineering properties shrink-swell, bearing capacity, erodibility
Soil Classification
•Old Scheme
–Pedocal Calcium soil arid and semi-arid, caliche
–Pedalfer Al Fe soil humid, no caliche
–Laterites: tropical soils of iron and aluminum oxides
•USDA Taxonomy (based on observable properties)
–*Histosol (hist = tissue) O Thick O horizon, minimal mineral horizons near surface.
–Aridisol arid soils
–Verstisol Vertically mixed soils, due to shrink-swell clays
–*Entisol (recent) Minimal development of horizons, weak A and O, no or minimal B
–*Inceptisol (inception of soil development) Well developed A, weak B
–*Spodosol (spod-= ash) much like an alfisol with a very pronounced E horizon. Often quartzose.
–**Mollisol (moll- =soft) has mollic horizon (a thick, black, soft horizon as found under prairies)
–**Alfisol (pedalfer soil) has an argillic (clay-rich) B horizon, and no mollic horizon
–Ultisol ultimate weathering Like an alfisol, but more so. Soluble nutrients are low or gone.
–Oxisol oxides only left (past ultisol) Also known as laterite. All soluble minerals, including quartz, are
gone, leaving only iron and aluminum oxides
** very common in Illinois
* common in Illinois
Soil Evolution
Initially, a soil is parent material at the surface. It is subjected to the soil forming
processes, changing the nature of the material.
• Organic matter accumulates, until a balance between deposition and decomposition is
• Weathering of minerals leads to
–release of soluble nutrients (K, Ca, Mg), which may be carried away by water
–formation of clays
• Translocation of clays downward and accumulation at water table (where speed
decreases) or where water is spread too thin to carry it onward
Example of till under grassland
1.Glacial till is exposed, not really a soil
2.Plants start to grow and organic matter accumulates as a weak A horizon Entisol
3.More organic matter accumulation to form a definite A horizon; weathering of
minerals and precipitation in soil causes a weak B horizon Inceptisol
4.Definite B horizon forms, much organic matter. Mollisol
5.Translocation of lots of clay from A horizon to B. Soluble minerals leached Alfisol
6.More translocation and leaching Ultisol
7.More translocation and leaching Oxisol
US Soils Fig. 5.18
Global Soil Regions USDA
Global Soil Regions FAO
Soils and History
•Cultures are constrained by their ability to produce food (among other
things), which is constrained by the soils and the plants.
–Irrigation civilizations, levees, flood soils
–Northern Europe was worthless until steel plow
•Soils are able to supply mineral nutrients for one of several reasons
–New material (glacial soils, volcanic soils, river floodplains)
–Influx of new material from wind or floods
–Influx or cycling of organic matter (rainforests)
•Some soils are just not good
–weathered too much (e.g., southern US, tropics)
–little initial nutrients (e.g., sandy soils in Michigan)
Weathering and Soils
• Weathering is the collection of processes that rocks undergo
to adjust to near-surface conditions, and includes
mechanical disintegration and chemical alterations. Most
primary igneous minerals weather to clay, some to quartz or
rust, and yield ions that go off in solution. Weathering rates
depend on starting materials, temperature, and availability
of water, oxygen, and acidity, and proceed much faster when
hot, wet, and lushly-vegetated.
• Soils form at the surface and the soil type depends on parent
material, time, topography, climate, and organisms. Soils
develop horizons, and these horizons along with other
properties are used to classify soils. Most soils evolve from
Entisols (raw parent material) towards a low-fertility Oxisol
due to leaching of soluble nutrients.